Commercial automotive coatings, that is, coatings for passenger cars and trucks, require many application and processing steps. Typically, a primer coat to provide corrosion resistance is applied by electrodeposition to the automotive body and then cured by heating in an oven. The automotive body may then be passed to a coating station where a primer surfacer is applied to provide a thick heavy pigmented coating that provides protection against road stone damage and also shields the electrodeposited primer layer from ultraviolet light that can deteriorate the primer layer. The primer surfacer layer is separately heat cured. Next a color-imparting basecoat is applied to the cured primer surfacer layer. Alternatively, in so-called “compact processes”, the primer surfacer layer and the separate curing step is eliminated and replaced with a coating that co-cures with the basecoat layer. The basecoat layer is typically derived from a water-based composition comprising a polymer, typically a (meth)acrylic polyol and a melamine curing agent. The basecoat layer is typically dehydrated at about 80-85° C. but not cured. An unpigmented transparent coat, called a clearcoat, is applied to the dehydrated basecoat (color plus clear coating). The clearcoat is a curable composition and the composite color plus clear coating cured in one step at about 140-150° C. This is the case even if the clearcoat is curable at lower temperatures such as with clearcoats that are based on hydroxyl-isocyanate curing because the basecoat composition with the melamine curing agent requires higher temperatures for curing.
However, a problem exists in that the basecoat layer is only dehydrated but not cured before application of the clearcoat. The uncured or partially cured basecoat does not have sufficient “hold out” properties, that is, solvents from the clearcoat can migrate into the basecoat adversely affecting pigment orientation in the basecoat. Also, lack of cure can adversely affect intercoat adhesion. The basecoat could be cured before application of the clearcoat but this would add another energy-consuming step in the coating process.
To overcome this problem, it has been suggested in U.S. application Ser. No. 13/780,540, filed 28 Feb. 2013 (PCT application Ser. No. 14/018,319, filed 25 Feb. 2014) to formulate the aqueous basecoat composition that cures through dehydration specifically using keto/aldo hydrazide curing mechanism resulting in a high degree of cure before the clearcoat is applied. Since the basecoat has a high degree of cure, this allows the clearcoat to be cured at a lower temperature.
The present invention provides an improvement to the above-mentioned basecoat composition by adding to the formulation a polymer that improves the chip and humidity resistance, as determined by the degree of blistering of the color plus clear coating, of the basecoat and of the composite base-clear coating.